text:
What is this sound from Airbus aircraft before take off?
When flying in an Airbus, I often (if not always) hear a weird sound before the aircraft starts to accelerate for taking off. From where exactly does this sound come from? Why doesn't it appear on other aircraft?

Hm, I wonder what sound you mean. The PTU is heard when starting engines, so during or just after push-back. I can't remember anything before the aircraft starts to accelerate. Can you find some video that would have that sound?

It's definitely the PTU as you can hear on this video or this other one. And I also do remember hearing this sound at the point just before the acceleration for take off.

That's the PTU, the Passenger Terrification Unit, which is designed to terrify the passengers and quiet them down by making loud, banging noises. It comes on every so often and can also be manually activated by a button in the cockpit.

According to Ask the Pilot:

Almost every frequent flyer has encountered this sound at one time or
  another. [...]
It happens on twin-engine Airbus models: the A320 series (includes the
  subvariants A319 and A321) and the larger A330. [...]
What you hear is a device called the power transfer unit, or PTU,
  which is designed to ensure adequate hydraulic pressures during
  single-engine operations. [...] Since it is activated only when the
  pressure falls below a certain level, the PTU cycles on and off, on
  and off, on and off.
[...]
Some Boeing aircraft also employ a PTU, but the operation is slightly
  different and it doesn’t bark like a dog.
Another noise peculiar to Airbus models is a shrill, prolonged whine
  heard at the gate prior to departure and again after landing. This is
  an electric hydraulic pump used to open and close the cargo doors.


text:
What's the difference between a two-blade prop and three-blade?
I read that a three blade can improve climb performance and decrease noise.  How does that work?

At the absolute simplest, compared to a two-bladed propeller, a slightly smaller three-bladed propeller can produce more thrust at a given RPM, improving climb performance. Because of the smaller diameter, the propeller tips actually travel slower than the equivalent two-bladed design, reducing noise inside and outside the airplane.
Hartzell Propeller has a great set of technical answers about propellers. Their explanation of why more blades produces less noise is pretty good:

A 2-blade propeller produces two pressure pulses per revolution, where a 3-blade propeller will produce three smaller pulses per revolution (for the same amount of total thrust) which is inherently smoother and therefore quieter. The 3-blade propeller will generally have a smaller diameter than the 2-blade propeller that it replaces, which also reduces the tip speed and noise.

Thanks for the answer!  if it produces more thrust at a given rpm, does the engine use more gas, or the same?

I imagine it would use more fuel as it's doing more work (producing more thrust). The Hartzell FAQ I linked mentions that two-bladed designs are the most efficient, but that more blades can produce more thrust and therefore can be advantageous.

A practical aspect using more blades is larger ground clearance.

@Garrison Neely: a simplification, but for a given engine fuel usage is more or less proportional to power output, as example HP or KW. At a given speed, power output is proportional to thrust.

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text:
Do pilots using an electronic flight bag need to carry paper charts?
If a pilot uses an electronic flight bag (say, an iPad with ForeFlight or WingX), are they required to carry paper charts as a backup?
Do different rules apply to operations under Parts 91, 121, and 135?

EFB application survey: http://softwarerecs.stackexchange.com/questions/11259/what-efb-application-do-you-recommend-for-aviation

A EFB is a legal replacement for paper charts and you are not required to carry backup if you are operating as part 91 single engine piston. Advisory Circular 120-76B is in reference to part 91F (Large and Turbine-Powered Multiengine Airplanes).
AC 91-78 is aimed at Part 91 operators, VFR or IFR and states that EFBs can be used in all phases of flight in lieu of paper.
A backup data source is suggested, but is not required. Note that this backup can be another electronic device.
See iPad Legal Briefing for Pilots.

Typically pilots using EFBs (which need to be approved by the FAA if being used for Part 121 or 135 flights) do not need to carry paper charts if, as Steve points out, you have at least two of them.  For Part 121, 135, and 91K there is usually a six month period after gaining EFB approval when the FAA requires paper backups in addition to the EFB's as a precaution.
A typical EFB program will have contingency procedures to use in the event of a failure or problem with one or more of the EFBs, and usually says that if one fails (and it can not be replaced or repaired) that you may not depart unless you have paper backups.
Lots of other good information about EFBs is available in Advisory Circular 120-76B and in Advisory Circular 91-78.

Advisory Circular 120-76B does not apply to part 91 single engine piston operators, but is intended for operators of part 121, 125, 135, or 91 subpart F (part 91F)(Large and Turbine-Powered Multiengine Airplanes).

@Magnetoz True!  Added the 91 Reference as well, however it doesn't change my answer.  :)

Yes, if you only carry one EFB, then you have to also carry paper charts as a backup.
See AC 120-76B.

a. Paper Data Removal. At least two operational EFBs are required to remove paper
  products that contain aeronautical charts, checklists, or other data required by the operating rules. The design of the EFB function requires that no single failure or
  common mode error may cause the loss of required aeronautical information.

If paper charts are "required operating information" If you would otherwise be required to carry a paper chart, then you either still have to carry one, or else carry two EFBs.
What about part 91 operators?

Aircraft operated under part 91, except for parts 91F and 91K, require no EFB authorization or compliance with this AC, provided the EFB does not replace any equipment or operating information required by the regulations.

Do you have a citation that states GA Part 91 operators are required to carry charts?

s/operators/pilots/

@egid: Haha, I don't think that your "typical" aviation person will understand your second comment.  :-)

@inafziger - Wouldn't be a Stack Network site without somebody casually dropping sed expressions in comments.

@SteveV. I'm still not 100% convinced that charts are considered "operating information required by the regulations", based on some previous conversations I've had about this.

Scratch that: I'm now 100% convinced that charts are not required operating information. FAA Aeronav has a FAQ about this. > "Not all pilots are required to carry a chart." "91.503..requires the pilot in command of large and multiengine airplanes to have charts." "Other operating sections of the FAR such as Part 121 and Part 135 operations have similar requirements."

Edited - thanks @egid!

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text:
Why are updraft carburetors standard in aviation?
My only detailed experience with carburetors is in aircraft. I'm pretty familiar with the principles behind float-type carbs, but I recently saw a schematic for a "downdraft carburetor" with a choke valve. This got me curious, so I did a little research and found that what I'm used to is an "updraft carburetor", and that (according to wikipedia) they fell out of fashion in the automotive industry in the 1930s.
Why is the updraft carburetor design so prevalent in aviation? Does an updraft carb actually have a choke valve?

Images below to provide a little context for those of us who are not engine experts!
Typical aviation carb:

Typical downdraft automotive carb:

I have no idea what an updraft carb is, but in my experience the answer to almost any question about "Why is X so prevalent in aviation" is reliability.

Yeah, that's a good point. I've added some images to show the differences (and some more context)

I can think of three reasons to use an updraft carburetor in a single-engine GA airplane:
Anti-flooding:
The float-bowl regulator in your carburetor is cheap and simple, but it is far from perfect under all conditions.  Short periods of zero-G or negative-G operation, caused by a quick dive, turbulence, or even bumping over uneven ground, can jiggle the float and spill liquid fuel into the venturi.
In a downdraft or sidedraft carburetor, this liquid falls into the intake manifold and wets the interior walls and may get into the cylinders, causing an over-rich mixture condition.  The engine is said to be "flooded" and will not run until the liquid is cleared out.
With an updraft carburetor, the spilled fuel tends to fall out before it can foul anything.
Gravity feed:
If your aircraft has fuel tanks higher than the carburetor when in cruise attitude, you may be able to remain under power if your fuel pump fails.  An updraft carburetor can be mounted lower in the engine compartment than sidedraft or downdraft types; in fact, it is usually under the engine.
Early automobiles did this to avoid the need for a fuel pump; fuel was fed by gravity from a fuel tank higher than the carburetor, e.g. just below the windshield.  The lower the position of the carburetor, the deeper the fuel tank could be.
Convenience:
If you look inside the cowling of most GA planes, you will see much more space under the engine block than above it.  That seems to be the best place for lots of auxiliary equipment and attachments, including the carburetor and air filter.

Of course, fuel injection would be far better than any carburetor... but that would require someone to actually design, build, test, and obtain certification for a new aircraft piston engine.

You get a lot of answers about shorter intake manifolds, less probability of flooding the cylinders if your carb floods, and many other things.
But to me, it comes down to one big issue: visibility. You need to mount your engine high enough that you have adequate prop clearance without having to have long, heavy landing gear struts. So you mount your engine fairly high on the airframe. Now do you want to have a carburetor sitting on top of that engine directly in the pilot's field of view? Why would you do that when you have all that space down below the engine?

Visibility is well documented in aircraft design text books.  The crankshaft places the center of mass of the engine, and for a single engine, there is customarily a windshield above the engine.  That leaves more space below the engine for accessories such as carburetors.

I dont know but when reading about the Northrop P-61 Black Widow WWII US night fighter I found this in Wikipedia :

XP-61 development
In March 1941, the Army/Navy Standardization Committee decided to
  standardize use of updraft carburetors across all U.S. military
  branches. The XP-61, designed with downdraft carburetors, faced an
  estimated minimum two-month redesign of the engine nacelle to bring
  the design into compliance. The committee later reversed the updraft
  carburetor standardization decision (the XP-61 program's predicament
  likely having little influence), preventing a potential setback in the
  XP-61's development

The fact that they made the decision to only use updraft carburettors, even though they later reversed the decision, indicates that there must be a good reason - maybe you can find the committee's notes on the subject.

Welcome to aviation.SE! This is interesting and relevant, but it doesn't really answer the question. If you're new to the site, I suggest you check out the [tour] and [help] to see how the Q&A format here works.


text:
Where is the taxiway between the Boeing factory and Paine Field airport?
How do the Boeing aircraft taxi to Paine Field (KPAE) from the Everett Factory? It looks like a 'road' connecting the airport and the factory. Do they use this?

You mean the bridge over the 526?

I don't see any other way they could get across.

I don't really get why you asked this? Are taxiway bridges not common? I always thought so, but maybe just because my home airport has one?

That "road" is in fact a taxilane, and marked as such on the Paine Field side (note the yellow lead lines marked at the bottom of the image below, you would find on a taxiway). It connects the Boeing plant to Paine Field's "Boeing Ramp" where final prep can be completed before the aircraft are flown off to wherever they're going.

The terminology is important here: The bridge acts as a taxilane (a non-movement area not under the control of ATC which connects various aircraft parking areas to the taxiways). Boeing coordinates their own movements between the main plant (north) and Paine Field's Boeing Ramp (south). (The bridge is also not depicted on the airport diagram and may not even have "taxilane" status - as far as ATC is concerned. It's just sort of "there".)

I fly out of KPAE. I can confirm that "the Boeing ramp" is nonmovement; they have their own controller and Paine Tower (well, Paine Ground) couldn't care less.

Hell's bells!  That is NARROW!   On a 787 the engines are fully outside the bridge edges eh?

The aircraft are moved from the factory to the airfield and paint shops on the airfield side over the bridge.
The photo shows a 787 Dreamliner being moved across the bridge.

Source: airlinereporter.com

Thanks guys, this Bridge over the 526 from the main factory to the Boeing Ramp seemed the only way across. By declaring it as a 'taxilane', and I presume they always use a tug for movements, and Ground movements people for Paine not interested until a/c wants to use official taxiways, works very well.

That's the photo I was looking for but couldn't find! (Google kept giving me photos of the unfinished fuselages crossing the North Bridge.)

@Geoff As far as I'm aware Boeing always uses a tug to move aircraft over the bridge to KPAE, though that's not because it's a taxilane (nor would it have to be the case if it's just an undesignated bit of "through-the-fence" access) - they could certainly move the planes under their own power if the engines are mounted, but that would likely cost more (fuel burn) than using the tug.

Is it safe to move a jet under it's own power over a narrow (compared to the aircraft) bridge like that? I would be worried about what impact jet blast from the engines could have.

I retired from Boeing and worked at the Everett plant for the majority of my career. We always moved the aircraft from the factory apron across the bridge to the paint hangars with a tug.
The aircraft are not fueled until ready for engine and other ground pre-flight tests. The engines are never fired up unless there are intake cages and blast fences in place. There are also FOD sweeps made to insure no foreign material gets pulled into the engine intakes. Furthermore, the “jet blast” would be destructive to property and the personal vehicles parked on the factory apron. In addition, it would require a certified individual to “drive” the aircraft under power.

Can you elaborate more, maybe with a diagram, so we know exactly where the said taxi way is located.

Fantastic answer, thanks for the real world info !

@vasin1987 , it's hard to see how much more clear this answer could be??

The bridge is the connecting taxiway from the main factory to the paint and finish facilities (the three large hangars on the right side of the photo), flight test and the customer delivery centers.  ‘Green’ airplanes are moved directly from final assembly to this area via the bridge.  This usually takes place in the early hours of the morning to reduce auto accidents caused by distracted motorists on the highway below.  It is quite a sight; a 747-400 crossing that bridge looks something akin to an elephant trying to walk a tightrope.


text:
How often should an engine be run, to stay mechanically sound?
It's winter time and many light aircraft are sitting idle. This can be detrimental to an engine's health.
What is a good rule of thumb for how often a piston engine should be exercised, and for how long?
If the aircraft cannot be flown, what is an acceptable alternative?

Remember it's not just the engine that gets miserable sitting in one place. Oleo struts can stick, wheel bearings might flat-spot, your tires/tubes will "take a set", that old mechanical coffee-grinder beacon may decide it's done grinding (Please die. I want to replace you.), etc... - the whole aircraft will be wanting some Tender Loving Care while it's stuck on the ramp!

All your plane's systems are happier when you're flying, including the pilot.
Ideally, you'd fly often in order to keep your engine happy (distributing clean oil throughout the system to protect against corrosion and heating it enough to drive off water from the crankcase).
As a rule of thumb Blackstone Labs (the oil analysis folks) consider piston engines "inactive" if they've had less than about 5 hours of flight time per month. Their threshold used to be 10 hours, so I personally try to fly at least 10 hours a month much when practical, but 5 at a minimum.
Blackstone has a LOT of data on aircraft engine wear patterns accumulated, and I generally trust their insight in this regard (after all they're not trying to sell me an engine, just asking me to pay them to play with my used oil).
When weather or other factors mean you can't fly enough to keep your engine "active" you can preserve it in accordance with the manufacturer's instructions, but let's be honest, most of us aren't going to "pickle" our engines for the winter. It's a LOT of work, and it means you can't go fly on those nice winter days we sometimes get.
In situations like this, it may be beneficial to ground-run your engine (say at least on a monthly basis) just to keep things lubricated and happy. Ground runs at a little below runup power or a fast idle are usually the order of the day here. Taxiing around is also probably a good idea, if practical, to keep your wheel bearings and related components moving -- maybe take your plane down to the runup pad and back.

When doing the ground run you want to observe a few guidelines to make sure what you're doing is actually beneficial to the engine:

Make sure you've got clean oil in the plane!
Any time you think the plane is going to be inactive you want to have good, clean oil in the engine (and you want to have done a thorough ground run or flight to spread that clean oil everywhere). This gives you optimal protection against corrosion.
Treat it like a flight.
Preflight the plane like you're taking it up, even if you're not.  You may not be flying much because of the weather, so there's a real chance your plane's systems will be in less-than-excellent shape. That thorough walk-around may catch a potentially expensive problem before it becomes a real issue.
Preheat if necessary.
Hand-in-hand with treating it like a real flight, you want to make sure you're not abusing your engine in the name of protecting it.
Get the engine oil to normal operating temperature.
Part of the point of the ground run is to drive off water that has condensed in your oil -- oil is good for metal parts, and water is bad for them.
If you don't get the engine oil up to normal middle-of-the-green-arc temperatures and keep it there for a good while (20-30 minutes at least) you may not be driving out the water, which can lead to a very unhappy engine.
If you don't get the oil up to operating temperature long enough you may wind up doing more harm than good! Combustion blow-by leaking around the piston rings contains water (and a bunch of other corrosive things your engine doesn't like), and really short ground runs that don't bring the engine to temperature are mainly just causing wear from the startup and dumping these nasty things into your oil. Your engine is not going to like that.
Run long enough to recharge your battery.
Watch your ammeter/load meter after startup, and make sure that your battery has been replenished before you shut down.
This means running your engine at a high enough RPM that the charging system can charge the battery (a fast idle is usually fine), and long enough that the battery can be charged (which isn't a problem, because if you were paying attention to #4 you've noticed we'll be running the engine a good while anyway).
Monitor your cylinder head temperatures.
If equipped with CHT probes, make sure you're not exceeding the CHT limits for your engine during the ground runs.  This goes hand in hand with #7 below.
Lean your mixture to avoid fouling your plugs.
You should be using less than 65% of your engine's max rated power for these ground runs -- in fact, you should probably be somewhere around 50-60%.  Rather than running full rich, pull back on the red knob and let the engine burn cleaner (but keep an eye out for the warning signs of detonation/preignition).
You probably only want to do this if you've got Cylinder Head Temperature readouts you trust -- when in doubt you can always run full rich (for the added cylinder cooling) and just clean your plugs.

Depending on the outside air temperature your ground run may last anywhere from 30 minutes to 90 minutes (I've not personally had one last longer than that, though it's theoretically possible I suppose) -- it all depends on how long #4 and #5 above take.

A final thought: I use ASL CamGuard in my oil. It goes in with every oil change.  Mike Busch (a smarter guy than me) swears by it, and there seems to be ample real-world-anecdotal evidence as well as a good amount of actual materials testing research that indicates it's a pretty decent corrosion inhibitor.
I've not had my plane/engine long enough to know if this treatment is effective or not, but it certainly doesn't seem to be hurting anything, and while it's a little expensive it's not "liquid gold" prices, so it strikes me as cheap enough insurance that you may as well dump it in the sump. This is especially true when you weigh it against the price of a bottom-end overhaul because you got pitted cam lobes or lifters...

An additional interesting read on the subject of corrosion, this old AvWeb article talks about the protective properties of CamGuard, as well as various oils with and without the additive mixed in. About half way down they talk about humidity cabinet test results, which is where my "once a month" above comes from -- In my case the oil I use (AeroShell 15W50) with CamGuard is good for about 25 days of protection in the lab, so really I should be running the engine to re-oil it every 20 or so days.

The articles I've read have suggested that engines should be run every week to prevent bore corrosion and that they should run for at least an hour to burn off all the condensation.  So, 4-5 hours a month again.  :-)

Ground running your engine is not good for it.
The objective is to get the oil hot enough to boil off any acidic water created by exhaust gas blow by (exhaust gas leaking around the piston rings and ending up in the crank case).  The oil temp should get up to 180 degrees (measured at the temperature sensor) which means it is much hotter down in the bearings etc. so the water is vaporized.  You can't do this on the ground without getting your cylinders too hot!  The only solution to this dilemma is to fly the plane whenever possible in the winter.
The other suggestion: to use CamGuard in the oil is a good one.  It keeps a layer of oil on the crank and camshafts longer than other products.  This lengthens the time you can let the plane sit safely between flights.

Yeah, I had always been under the impression that ground runs are, if not harmful, then certainly not helpful when it comes to maintaining an engine. However, if all you're concerned about is keeping your battery charged, they will certainly do that.

Flying the plane is definitely the preferred way to exercise the engine (and the other moving parts). Barring that ideally we'd pickle the engines in dry storage, but that's a tall order with no hangar. Like you pointed out the hard part of ground-running an engine is to get the oil temperature up without redlining the cylinder head temperatures - in my experience nigh-impossible with a static run (though I have done it with a runup and a brisk taxi-tour of the field in a pinch).


text:
How can one determine if a Service Bulletin is still applicable?
I've seen the occasional Service Bulletin that is marked Obsolete, and others that Supersede SB-123. If I'm a pilot or A&P looking through logbooks, what resources are available to determine if a given SB still applies to the aircraft?
For example, American SB-131 applies to AA-1, AA-1A, and AA-5 aircraft. However, this scan (pdf) seems to suggest that it's obsolete, and the contents are now in the service manual.

I can't speak to the FARs, but based on the issue date of 3 May, 1972, and This information will be added to the next subsequent revision of each applicable Service Manual., I think it's safe to say that this service bulletin is Obsolete.

Yeah, I got that much - but I'm trying to determine how we'd be able to figure this out in other situations. Then again, when it comes to the FAA, it's rarely safe to assume anything. :)

I'm trying to verify but I believe the manufacturer has to keep records about which SB's apply to which airframe by serial number. Obviously this gets more complicated when manufacturers go out of business but you should be able to call up any manufacturer with a serial number and ask for the active service bulletin numbers.

Yeah, in several cases that's hard as the manufacturer is years out of business. I guess you'd have to track down the type cert holder.

This FAA AC outlines how SB's should be drafted. It also says that the Design Approval Holder should make available an easy way to search the SB's

A DAH should develop a robust SB/AD tracking and management system
using the following process to ensure overlaps and conflicts between
new and existing SB actions are identified and addressed: a. Search
capabilities should include the following primary areas:
(1)
Make/model or part number of affected product(s),
(2) Major aircraft
elements (e.g., engine, strut, wing, etc.),
(3) ATA code,
(4)
Maintenance zones,
(5) Service information (e.g., service bulletin
number),
(6) Airworthiness limitation sections,
(7) Previously issued
ADs, and
(8) Planned ADs (i.e., action(s) that the FAA proposes to
mandate in an AD).

This puts the ownership on the company a bit.
The FAA makes all current bulletins available here so you can check if they still list it as current.
There is also a bit of duality between service bulletins and Air worthiness directives. SB's are issued by the company if they feel there is a safety hazard to the aircraft. Most importantly, unless they are issued alongside an AD they may not be mandatory. You can check out this blog post that goes into it a bit more.

There are two issues here: the first would be the status of the SB itself, and whether the work described in the service bulletin has been competed. The link in Dave's answer links to an FAA website the lists all special airworthiness information bulletins, and SAIBs are not the same as SBs. This problems becomes more challenging when the OEM is not actively creating and/or maintaining the maintenance and service documentation. If the OEM still is open for business, i.e. Cessna, Robinson, Boeing, etc., they usually have a method to subscribe for updated information. The FAA may mandate an easy way to get that information, but that does not mean it is inexpensive. It is through these subscriptions that you get the latest information. For aircraft that no longer have an OEM created maintenance and service documentation, that becomes a challenge. If in doubt, refer to http://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_20-176A.pdf.


text:
How does one reset an attitude indicator that doesn't have a cage?
I was up flying today – VFR – and the attitude indicator tumbled. It's a 6-year-old RC Allen 22-7, without the ability to cage, so I just wound up letting it be all off-kilter until things stabilized... which took almost an hour. The vacuum was in the green the entire flight, and at no point was I doing any wild maneuvering.
Is there anything I could have done to accelerate the process? Could I have caused damage by not landing and shutting down?

If it took an hour to stabilize, I'd certainly talk to maintenance about it. The chance of survival in an AI failure is 1 in 20...

That's definitely happening - but I realized I didn't know the process (or expectations) for older model AIs! Maybe the process is to replace it with a newer one.

@kevin I'm not doubting you, but do you have a reference for the odds, or is it an educated guess?

@CGCampbell It's a stat mentioned in one of the AOPA seminars. They mentioned several emergencies : VFR into IMC, AI failure, engine failure after takeoff, smoke in the cockpit. The speaker is a guy who has 6000 hours in a 172. I don't remember which one, but it's on YouTube.

@kevin No problem in VFR though.  Odds of survival, 100%

Only a mechanical inspection is likely to give you a definitive answer for why the AI tumbled, but typically it'll happen for one of two reasons: either the vacuum pump has decided to take a break, in which case the AI gyros slow down and will eventually no longer hold the instrument up, or you've exceeded the instrument's pitch and/or bank limits and caused it to tumble.
In the case of the former, there's not much you can do in a stock -172 about that. Section 3 in the PIM just tells to verify that the vacuum pump is operating with normal parameters (green arc is 4.5-5.5 in.hg). I've seen PA-28Rs with electric vacuum pumps that served as standbys to the engine-driven ones, but never seen one in a -172, not even as an STC mod. You could always try increasing the throttle; pumps in older aircraft will sometimes fail to perform as expected at lower throttle settings, but there's no guarantee there.
The latter case can happen during maneuvering flight if you exceed +/-30 degrees of pitch and 60 degrees of bank on a non-acrobatic AI. This can inadvertently happen when certain training maneuvers such as power-on or cross-controlled stalls are performed... enthusiastically. I've also seen tumbling occur during spin training. And then there was the Husky we used for banner-towing and which was pitched to 45 degrees on a regular basis during banner picks (waivers are wonderful things). The AI tumbled so regularly, the owner gave up on having mechanics reset it.
But yea, should this happen to you again, run any checklists that might exist on the topic (either directly concerned with the AI or with the vacuum pump system), and switch to partial panel ops if necessary.

The pump was in the green (and is plumbed off the back of the AI, which is actually a 6-year-old RC Allen 22-7), and there was no unusual maneuvering, which is partly why I was surprised. It's been ok on a subsequent flight so who knows.

That is unusual, but probably it's not something would warrant a more detailed inspection, especially if it recovered okay and worked subsequently, so yea.

The (now defunct) club I used to fly at had 5 C172P aircraft, and as I recall all of them had electric standby vacuum pumps.

@FredLarson do you remember if this was a stock configuration on the P model or an after-market mod?

@habu: I'm not sure I ever knew which it was. Might have been an STC add-on. But I have a C172P pilot manual at home. I'll see if it mentions anything about it.

I finally remembered to look it up. There's a supplemental section for an electric standby vacuum pump. It's also mentioned as optional equipment in the section about the pitot-static system.

You indeed have some kind of failure.  Except for performing aerobatics, there is nothing a pilot can do to harm the gyro.
The AI itself is probably failing.  But even though vacuum shows green, there's an unlikely chance the instrument itself is not getting adequate suction.  The vacuum lines should be checked at least visually before sending the unit in for repair.
Often times when a gyro starts to fail, you will notice it slow to stabilize when first starting the engine.  A healthy attitude indicator will become erect almost immediately. If it wobbles for more than a short moment, be suspicious.  Also become suspicious when a gyro become loud or whines.  You should be able to hear the gyro if you get close, but modern healthy gyros are pretty darned quiet.


text:
To a pilot, does turbocharging differ from supercharging?
When it comes to operating an aircraft, what are the practical differences between a turbocharged engine and a supercharged engine? I'm aware of the mechanics - turbochargers being exhaust-driven, while superchargers are mechanically driven - so I'm looking for the differences in how pilots need to treat the systems:

Do power settings need to be set differently?
Are temperatures more of a concern with superchargers?
Is it easier or harder to overboost a supercharger? If you do, are there waste gates?
Do engines with superchargers need a cool-down period before being shut off?
Any other operational differences that would be meaningful for pilots to know.

Similarly, what are the ownership implications of any differences? Does one cost more to maintain than the other due to the way they are constructed? Will a supercharged engine have a lower TBO because of the mechanical aspects of the system, or will it be the same as a turbocharged engine?

I have some of the answers:
•Is it easier or harder to overboost a supercharger? If you do, are there waste gates?
This is an important point.  Waste gates, when opened, allow exhaust to bypass the turbocharger, reducing manifold pressure by slowing down the turbo.  Older turbos had manually adjusted waste gates, and even fixed waste gates.  The pilot would have to monitor boost pressure and adjust the waste gate to avoid overboosting the engine.  Newer turbos have automatic waste gates.  Superchargers, being belt or gear driven, do not have waste gates as the supercharger is not dependent on exhaust gas flow.  I do not know if they have manifold pressure releif valves in case of overpressure but I doubt they would.
So a key difference in a pilot's operation of the engine is that in a manual waste gate engine, the manifold pressure gauge needs to be scanned frequently and the waste gate adjusted accordingly.   Less important in an automatic waste gate or supercharged engine.
•Do engines with superchargers need a cool-down period before being shut off?
No.  The cooldown period in turbochargers is necessary to cool down the bearings in the turbocharger after high cruise power settings.  Idling on the ground allows cooler oil to flow through the bearings, removing excess heat.  Look at your exhaust gas temperature gauge - this is the temperature of the gasses flowing through your turbocharger.  The oil present in the turbocharger bearings will "coke", or turn into a solid at that temperature.  Upon landing, a slow taxi to the ramp will usually provide sufficient cool down time.  Usually!  Time this and don't forget to cool down as recommended.
Superchargers are geared to the engine (or belted) and do not use the hot exhaust as their source of power.  So even though they can get quite hot, they don't approach the heat of the exhaust gasses.
•Any other operational differences that would be meaningful for pilots to know.
This is not really an operational difference, but in addition to turbocharged and supercharged engines, there are also turbonormalized engines.  TN engines are turbocharged engines with their boost limited to atmospheric pressure at sea level.  Say 30 inches, not the 40 inches you might expect from a turbocharged engine.  The TN engine permits sea level performance up to a critical altitude, above which the TN engine cannot maintain sea level manifold pressure.  In other words, performance at a sea level takeoff will not be materially different from that of a naturally aspirated engine, but as you climb, the power will not decline as you climb.  So your Leadville CO takeoff will be the envy of the stock C-172 crowd.

This is really close to what I'm looking for... do you happen to know anything about the maintenance costs of each? Turbos strike me as possibly having lower recurring maintenance but higher replacement costs?

It's more or less the same as in a car. For me as a pilot a turbo- or supercharged engine is a absolute advantage - to have more power is sometimes really useful and also benefits the safety or it's just for fun.
For everyone whos not sure about the working principle of these systems or the difference between a turbocharger and a supercharger I will give a short description:
Both are compressors installed in the manifold wich are used to get more air or air-fuel mixture into the engine or simply raise the density, which especially at higher altitudes provides some extra power. The difference between both is the way the compressors are driven.
A Turbocharger is driven by a turbine in the exhaust. That's very efficient because it uses some of the ~ 70% wasted energy in a piston engine but also creates some back pressure on the exhaust.
A supercharger is like other auxiliaries, e.g. the engine fuel pump, driven by the engine it self wich of cause costs a bit of engine efficiency but all in all the engine benefits from the installation. The great advantage compared to a turbocharger is that a supercharger works fine at all RPMs while a turbocharger needs some higher exhaust pressure to start working properly which makes it effective only at high power settings.
As a pilot you don't need to take care of some additional switches but must of cause treat your engine a little bit different (higher risk to overpower). Do not use intermittent throttle position during take off or climb - allways go full forward. The use of intermittent throttle positions only causes your engine to work at higher load and non optimal cooling for longer time. If RPM exceedslimits reduce the Prop pitch carefully - rapidly lowering the RPM can cause severe damage to the engine! If you are used to lean mixture on the ground, stick to it your engine (the spark plugs) will be thankful but go full mixture for take off, even at some higher altitudes to achive maximum internal cooling of the cylinders - monitor the temperatures. These are some general suggestions based on my experience and knowledge, please refer always to the manual!
Another risk I can thin of is, that you might be not used to flying at higher altitudes or VFR 'over the tops'. Try both with an instructor or experienced pilot first before doing it solo. You might be healthy and never had problems with your medical, anyway you should be able to lear how your body behaves. Also you might not be used to flying without the normal horizon - this might be lost when descending or climbing in VMC but in close vicinity of clouds. It's not my intention to consider you as unexperienced but I like to write this answer in a way, that it's helpful for everyone whos interested in.
Operator or owner of an aircraft should not only consider the advantages of forced ignition. Before I've edited this answer I wrote, that piston engines are nlt developed to be turbo/supercharged. This way to say it was obviously not really helpfull for you, sorry. What I want you to think of are the additional loads acting on the engine. You have high pressure within the cylinders,  even some back pressure on the exhaust (turbocharger only) or additional friction to drive the supercharger. The engines whitch are equipped with forced ignition or those where turbochargers are available fore of cause are built to handel these forces but I still consider them to effect the life of those engines. So there are definitely more extra costs then the purchase of the equipment.

I'm not really looking for how either system works; I'm interested in what it means from the pilot's perspective in the cockpit, or the owner's during maintenance. You say a couple of things that I'd love for you to expand on ("piston engines are not really developed for being turbo- or supercharged", which I find hard to believe, and "a turbocharged engine can also raise the maintenance costs of the engine", which I'm very interested in).

It was not my intention to teach you things you already know, sorry for that. Anyway I didn't mind the time to give a short and basic description for anybody whos interested in this question but not really sure about. I will be happy to give you a some more detailed insight to my thoughts later this day and edit my answer. If you feel like you're waiting too long, please feel free to remeber me with another comment. Anybody else who things he's sharing my thoughts, please also don't hesitate to add some details to my answer or share your point of view-of cause other sights are welcome as well.


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Becoming a CFI in the USA as a non-US citizen
This question is not about aviation itself, but rather about job opportunities in aviation.
It kind of goes into law a little, maybe somebody has some experience with this issue though:
Being from Germany, I'd like to become a FAA CFI. This itself is not a huge problem, but working as a CFI in the US is one, as I would need a citizenship or a green card to work there... or would I not?
Does anybody know anyone who is a CFI or works as a pilot in the US and is not a US-citizen?
Or does anybody have an idea of how to work as a CFI without a green card or citizenship?
Knowing this isn't pure aviation, this still appears to me as one of the best places to ask, you all know that experience can not be out ruled!

This is really a question about employment law, not about aviation. To legally work in the US as a foreigner you need a green card or an appropriate visa (not all visas allow you to work!), but there are lots of rules about how to get them and it all depends on your personal situation. I suggest you try expatriates.SE, they seem to have a lot of questions like this.

You do not need to be a citizen to be a U.S. CFI.
You need to have US ratings, so you would have to have all your German licenses/ratings converted, which takes some time.
You have to be absolutely FLUENT in both spoken and written English.
If you want to charge students money for your services, you have to have US residency or other work visa authorization. You do not need a work visa if you train students for free.

I know this question is closed, but I wanted to point out that the question wasn't about 'can a foreigner become an FAA CFI?' (he specifically says he knows that's possible) — it's 'can a foreigner work as a CFI in the US?'.

@egid Well, that would be a legal question that has nothing to do with aviation. As I pointed out in my answer, you have to have a work authorization to do ANY job in the US.

Yes. Exactly. That's why it got closed. :)


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I hold both a Sport Pilot License and a Student Pilot certificate. Can I do my BFR in a C-172?
FAR § 61.56.(c).(1) states that every two years I must have

Accomplished a flight review given in an aircraft for which that pilot is rated by an authorized instructor.

The answer to my question hinges on the exact meaning of the word "rated". If I've been signed off to solo in a C-172, am I "rated" for that aircraft as far as this regulation is concerned?

I believe that because you cannot use your Sport privileges to act as PIC in a non-sport aircraft, you cannot perform a BFR in one.

@egid I believe you don't need to be PIC during a flight review, the CFI can be PIC. Otherwise it would be impossible to do one if your previous flight review had lapsed.

I'm looking it up. I'm fairly certain you need to be able to act as PIC ("an aircraft for which that pilot is rated") - otherwise one could get a flight review in a helicopter for their PPL SEL.

And I specifically did say that he cannot act as PIC - but you're right, I should have said "in general" as it's not possible to be legal PIC (although one could be sole manipulator) for the completion of a flight review.

@egid Apparently the FAA says that you only need one flight review: "For example, a pilot who holds a private pilot certificate with an ASEL rating and a commercial balloon certificate may take a flight review in either aircraft and will have met the requirements of the rule for both". That surprised me.

Yeah, that's normal. But I'm pretty sure you need to do the review in an airplane on your cert. still looking!

@egid: That's the thing, though, as a solo student, I can act as PIC of a 172 (although not when the instructor is aboard), which is the airplane on my student cert. So arguably, the solo endorsement makes me "rated" for the aircraft.

@ThrowawayAccount3Million Yeah, except a solo student pilot is explicitly excluded from the need for a Flight Review per 61.56(g), so I'd disagree with you on that part. You can't fly the 172 using your Sport privileges, therefore you cannot use it to perform a flight review.

Nope, sorry! If you need a flight review, use an LSA you have been endorsed for.
§61.56(c)(1) states that a flight review must be:

Accomplished [...] in an aircraft for which that pilot is rated

According to the AOPA (note: this article is specifically targeted at non-Sport pilots):

Rated is interpreted as category and class [emphasis mine]

And in another article, the AOPA continues:

Sport pilot certificates will be issued without category/class designation

...and just for the final nail in the coffin, here's the FAA itself (thanks, @Pondlife!), in AC 61-98B:

A pilot who holds only a sport pilot certificate may only take a flight review in a light sport aircraft for which he or she holds an operational privilege. For example, a sport pilot who holds airplane privileges could not take the flight review in a Cessna 172 since that airplane is not a light sport airplane and he or she does not hold operating privileges for that airplane.

Long story short, this all means that no, a Sport Pilot cannot perform a flight review in a non-LSA type. Because you require specific logbook endorsements in make/model for category/class/speed (an LSA like a Sting S3 vs Airplane Single-Engine Land), you are not rated to fly a Cessna 172, for example. Your solo endorsement isn't a rating, as it isn't part of a certificate - it's an endorsement.
Once you become a Private Pilot, you'll be able to do a flight review in almost any ASEL type, including all LSAs because you're suddenly rated for the Airplane category and Single Engine Land class, with no light-sport-only limitation.
As far as I can tell, your student pilot certificate is actually invalid. It's been superseded by your Sport Pilot certificate and you are now adding privileges to that certificate. As such, it doesn't really matter that you've been endorsed for solo in a 172; you aren't rated to fly one in the same way you are for an LSA.

I'm attempting to nail down if the FAA considers LSA it's own category.

FWIW, my SPL says "AIRPLANE SINGLE-ENGINE LAND" on the back, and during training I was told that LSAs can have the same categories and classes as other aircraft (except where the LSA definition prohibits it, so there can be and are LSA rotorcraft, but there can't be an LSA twin), which corresponded with the correct answers on the written test.

Also, I don't see how a Third Class Medical And Student Pilot Certificate can be superseded by a certificate that doesn't confer the privileges of a student pilot certificate. It clearly doesn't add privileges to the SPL, since by definition an SPL limits you to flying day VFR below the clouds in an LSA, while allowing you to fly within those limitations without a medical.

I'm gonna have to call my FSDO and see what they say.

@ThrowawayAccount3Million: a certificated pilot doesn't need a student cert to add new categories or classes. Otherwise a PPL SEL would need a student cert to add rotorcraft, for example, and that's not the case. The same should hold true for sport. A Sport Pilot needs a medical but not a student cert when adding ratings.

I suppose I technically already have my 10 solo hours, 150nm cross country, and 3 full-stop landings at a towered airport, etc. all flown in  LSAs, so after doing night, instrument, and other dual stuff I could move straight to the checkride without ever having flown a 172 without an instructor on board. So you're saying that means I can't be endorsed to solo a 172 before doing a checkride?

I'm not saying that at all. The endorsement just doesn't go on your student certificate. :)

As mentioned above, but hard to see - once you pass any check ride and you are a pilot (sport, recreation, private, commercial or airline transport) so no longer a Student Pilot. You are a Pilot; different rules apply for adding a new category/class to your Pilot Certificate.
A Flight Review (Hasn't been a "BFR" since 1997. The FAA's preferred term is "Flight Review," read the Advisory Circular!) must take place in an aircraft for which the pilot is rated. 61.56 provides useful alternatives. One of these is training via the FAA's Wings program.
In the OP's case, three dual flights in the 172, combined with some online training, could provide credit for the flight review. Just a little planning/coordination with the instructor is required.

According to Flying Magazine January 2010, it sounds like, yes you can:

Q: I'm a private pilot flying at the Sport Pilot level. Can I take my
  flight review in the Cessna 172 that's available at my local FBO?
A: The requirements for a flight review are called out in 14 CFR
  61.56. The regulation requires the flight review to be accomplished in an aircraft for which the pilot is rated. Since you are a private
  pilot you are rated to fly the Cessna 172 (assuming you have an
  Airplane Single Engine Land rating on your pilot certificate), so this
  would meet the requirement of the regulation. Since you don't hold an
  FAA medical certificate, you would not be authorized to act as pilot
  in command (PIC) during the flight portion of the review, but Sec.
  61.56 does not contain a requirement for the pilot taking the flight review to hold a medical certificate or to act as PIC, so this is not
  a problem. The flight instructor performing the flight review will act
  as PIC during the flight.

The OP is not a Private Pilot though, he is a Sport Pilot. "Since you are a private pilot you are rated to fly the Cessna 172..." The OP is not "rated" to fly it, the OP can fly LSA's, and has an endorsement to fly 172's solo. A solo flight is simply an instructional flight, under supervision of a flight instructor but on which the flight instructor is not present. He cannot fly a 172 for any other reason than an instructional flight.

If you currently hold a Sport Pilot Certificate or other grade of Pilot Certificate, your Student Pilot Certificate is no longer valid in the eyes of the FAA and the law.  Therefore §61.56(c)(1) governs this and the flight review must be conducted aboard an aircraft which the pilot is appropriately rated to serve as PIC aboard.


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Are you required to have constant view of the runway when circling?
When you're circling, it has been said that you are required to keep view of the runway at all times below the MDA. However, sometimes this can be impossible due to issues with visibility, such as being banked to turn around for final, or because a flock of birds are circling between the airport and your aircraft. Are you required to execute the missed approach if this occurs or not?

A "circle-to-land" maneuver is basically a low-altitude traffic pattern - albeit one that you arrived at by being guided in on some instrument approach. The magic that makes this low (and possibly abbreviated) pattern possible is that the runway ends at airports with circling approaches have protected (obstacle free) airspace around them, which looks something like this:

(A more detailed explanation of the size of the circling approach area can be found on this question.)
Since it's basically a traffic pattern my understanding is you treat it as such, and that as long as you stay in visual contact with the airport and within the protected airspace (by visually judging your distance from the field, and knowing your altitude) you're unlikely to hit anything, and you can do all your usual pattern maneuvering as long as you can ***visually* be sure you're within the pretty blue part of the image above (your GPS cant help you here).

To see what this means for an example approach with some turns, consider my liberally-stolen and slightly-marked-up diagram below.

If you lose contact with your intended runway end for a few seconds because of the turn/bank required at point "A" that's OK: as long as you can still maintain visual contact with the airport/runway environment when you roll out (say by looking out the back window), and as long as you can remain clear of clouds, and can maintain visual clearance from any potential obstacles you may continue the approach.
If you're flying out and on the leg from "A" to "B" and all of a sudden you look back out the window and the airport is gone in the haze and clouds, you need to go missed: You've lost the airport/runway environment, and can no longer judge your distance to stay in the "safe" airspace.
Similarly if you're in a high-wing aircraft and your turns at "B" and "C" briefly obscure the airport while you're turning you can still continue the approach - if you've any doubt where the airport is just pick up the wing like you would in a VFR pattern (and if the airport isn't there like it's supposed to be go missed!)

Regarding your bird scenario, that's an interesting one that I don't think is covered in any diagrams, but if a flock of birds causes you to maneuver in such a way that you lose contact with the airport (and you can't just pick up a wing and have it be right where you expected it to be) you're obviously going to have to abandon the approach and go missed. It's the equivalent of the airport disappearing in the haze on that A-to-B leg.
Honestly though, if a flock of birds shows up and hangs out in a way that requires you to start maneuvering around them while you're trying to shoot a circling approach in real IFR you may want to just go missed anyway (given the odds this probably won't cost you too many approaches).
In a real world situation with poor visibility, possibly precipitation, and likely a cloud ceiling really close to your altitude, do you really want to try to be maneuvering down low to avoid some stupid bird that couldn't even be bothered to call the tower?
Wouldn't you feel much better pointing the aircraft in a generally "upward" direction and following the nice friendly needle again for a little while?
I'm just starting my IFR training, but I think I'd feel better with that option :-)

According to 14 CFR 91.175(e) you must go missed...

Whenever an identifiable part of the airport is not distinctly visible
  to the pilot during a circling maneuver at or above MDA, unless the
  inability to see an identifiable part of the airport results only from
  a normal bank of the aircraft during the circling approach.

I added the emphasis about the MDA, because 91.175(c) says that below the MDA you're required to have the runway environment in sight.
This FAA paper on approaches to MDA/DH has more information and also states that the airport must be in sight:

You must have visual contact with the airport before attempting a
  circle-to-land maneuver, and you must maintain visual contact with the
  airport at all times while circling to land.

The AIM 5-4-21 explains how to get established on the missed approach:

To become established on the prescribed missed approach course, the pilot should make an initial climbing turn toward the landing runway and continue the turn until established on the missed approach course

Good find on that FAA paper - I was looking for it last night but my Google-Fu was weak :(

"visual reference" does not necessarily mean the runway does it? If you can see anything, and you know where the runway is in relation to what you're seeing (e.g. the control tower, or a radar dish), wouldn't that count as still having a visual reference?

@roe FAR 91.175 defines the allowed visual references and they're all part of the runway environment, so I believe the tower itself wouldn't count.

@roe note that all as Pondlife said the elements you can use as references are part of the runway environment, but they're spaced out along the whole length of the runway -- that is you don't need to have the runway end you're going to land on in sight at all times, you just need at least one of those elements to continue your approach (and it seems like it would be pretty darn hard to lose them all and still be in a situation where you'd want to continue the attempt to land).

@pondlife I'm not sure that the visual references to continue below DA in actual apply here, to a non precision circling approach. I've always understood this requirement to include the airport environment.

@egid After re-reading 91.175e(2), I think you're right, the airport must be in sight but not necessarily the runway. It says you must go missed "Whenever an identifiable part of the airport is not distinctly visible to the pilot during a circling maneuver at or above MDA, unless the inability to see an identifiable part of the airport results only from a normal bank of the aircraft during the circling approach". So it's airport at or above DA and runway below, as far as I can tell. Good catch, I'll update my answer.


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How can I estimate the minimum runway needed to take off with a hang glider?
I'm interested in short, or trick, take-offs - such as from platforms, tall trees, etc.
I think that I should have

a wind speed and direction measure
an understanding of my wing surface area

This will let me add to my intuition from regular launches (from sites with known-good launch conditions), and estimate how much velocity I need to add via a run / push.

The methodology to count the wind measure seem a bit more grey, right now.
I can have a sense of the wind where I am, but it may quickly change beyond my launch site.  Since I'm considering how to launch from a stationary position, it would be good to feed my senses / calculations more data like this.

You can objectively measure the wind speed with wind speed meter (anemometer).

This is a rather open-ended question.  It is possible to do a cliff launch where you simply step off the cliff, holding the glider pointing steeply downwards.  In this case your required takeoff run is zero.  I'm not sure whether or not this can be safely done in no wind-- perhaps off a platform projecting far in front of the cliff like a diving board?  So a key question is, does your question intend the glider to be up to a normal flying airspeed before reaching the end of the launch run, or is it allowed to gain speed by falling a ways?

What you need is enough lift to remain in the air. For that, the maximum lift of your hang glider needs to exceed the total weight.
When you know the total weight of the glider (you included), you can derive the theoretical minimum airspeed from the lift formula:
$$L_{max}=\frac12{\rho}V^2C_{max}S$$
$$V_{min}=\sqrt{\frac{2W}{{\rho}C_{max}S}}$$
Where:

$L$ = lift
$\rho$ = air density
$V$ = air speed
$C_{max}$ = maximum lift coefficient
$S$ = wing surface
$W$ = weight

Now you have to know your maximum lift coefficient. Suppose this is 1.2, and for safety and initial manoeuvring, let's add a margin of 40% to the weight.
Let's say your mass is 85 kg, and the glider has a mass of 35 kg. The total weight would be 120 kg — that is about 1,177N weight, which for the sake of safety margin is increased to 1,650N.
I'll estimate the surface of the wing to be $16m^2$. Assume a normal day, the air density will be about $1.225kg/m^3$.
Filling in the numbers is the formula:
$$V_{min}=\sqrt{\frac{2\times1650}{1.225\times1.2\times16}}\approx 12 \textrm{ m/s}$$
Any wind speed you get you can subtract from this figure: what is left you have to achieve by pushing, running or diving.

@Qantas94Heavy Thanks for editing, it is a major improvement.

Hopefully it's even better now ;)

An airspeed indicator is probably the most critical
instrument one can have in any type of aircraft they happen to be flying.  It can save your life.  Airspeed should never be "grey". While understanding the theory is important, far more so is the performance in the air, in other words, thoroughly testing AT ALTITUDE
the stall speed and characteristics, and speaking with people experienced in your type.  And, as previously suggested, build in a safety margin and adhere to in when getting close to the ground.


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Does the Piper PA-28 Cherokee wing have laminar flow behavior?
I've heard (and read) that the PA-28 series has a laminar flow wing:

The Piper had a very thick airfoil that used laminar flow to assure that lift would be lost very slowly up to the stall.

People seem to pitch the Cherokee's wing as some sort of amazing safety and performance improvement. I've always understood that laminar flow wings had an abrupt, harsh stall, but offered reduced drag in cruise. I have a ton of early-Cherokee time on the 'Hershey Bar' wing, and its stall behavior is gentle and predictable, while its cruise speed is almost knot-for-knot on par with a Cessna 172 of comparable power.
Is the PA-28 laminar flow wing just a marketing gimmick? If one were to put a Cherokee wing in a wind tunnel, would we see laminar flow?

Short answer: Yes. You would see laminar flow.
Longer answer: It depends. In case of the wind tunnel it depends on the laminarity of the wind tunnel: Older designs had too much turbulence to allow for much laminarity. It was precisely for that reason that laminar flow was not well understood in the first decades of aviation.
The airfoil on the Cherokee is the NACA $65_2415$, and its Reynolds number in normal operation is in the single digit millions. The laminar bucket is ideally between the lift coefficients from 0.2 to 0.6, and the Reynolds number is low enough to allow for laminar flow. When you approach stall, however, the upper side has a pronounced suction peak near the nose which lets the flow rapidly transition to turbulent flow right after the peak. Laminar flow can only be found on the lower side (if surface roughness is low enough).
The pressure distribution over the rear 50% of the airfoil shows a constant pressure rise, so the onset of stall is marked by flow separation which starts at the trailing edge and slowly marches forward with increasing angle of attack. This leaves the suction area in the forward part unperturbed and keeps lift nearly constant around stall.
Contrast this with the stall behavior of the 5-digit NACA series, which can only be described as nasty. Stall is marked by flow separation right past the suction peak at the nose, and the lift coefficient suddenly drops by at least 0.2 counts when the airfoil stalls. Only some laminar airfoils have a similarly harsh stall behavior; this happens mostly when their designers were too ambitious and achieved the pressure rise with a Stratford pressure distribution. This is a distribution which keeps the separation margin constant along its length, and at stall it is exceeded everywhere at once, creating a sudden, large separation.
A well-designed laminar airfoil has a gentle stall behavior. Its lift coefficients with laminar flow are normally quite a bit away from stall, and its stall characteristics can be tailored independently from the region with laminar flow. Using a NACA $65_2415$ instead of a NACA 23015 is not a marketing gimmick, but a sound choice.
Regarding the performance: As long as the plane has a fixed landing gear, the laminarity of the wing will have little effect on top speed - the parasitic drag of all those things sticking out from the airframe will dominate drag. In addition, the higher the Reynolds number in flight is, the shorter the laminar flow will be before it transitions to turbulent flow, so the effect is much reduced at higher speed. Also, the way the wing is built makes a lot of difference, because every rivet head and every gap will create a wedge of turbulent flow behind it.

Super interesting stuff. Thanks, Peter!

It would be interesting to compare the real-world performance of a "perfect" 65(2)-415 airfoil (made with composites and molded with a continuous surface) vs. the PA-28's rivets-and-screws-all-over-the-top-surface construction. I would instinctively expect slight performance improvements (heading toward what you can get in the AA-1/AA-5 with bonded wings), though like Peter said form drag is going to be the dominant factor in performance here.


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How does a supersonic jet engine differ from a subsonic jet engine?
All jet engines have the same method of air flowing into the engine, which is compressed and ignited.
What can engine designers do to make an engine design create more thrust? For example, what is different when comparing the Olympus engines on the supersonic Concorde to the CFM56 engines on a 737?

If you are asking about engines in particular, than this question is simply duplicate of http://aviation.stackexchange.com/q/9066/524 (the second answer gives more details than the accepted one). Also relevant explanation in this answer to more specific question about inlets (I considered writing an answer here, but Peter's answers provide such great detail that linking them here is probably better).

I've tried to rewrite the question in a way that could gather answers that are a) relevant to OP and b) educational in general.

Thrust is air mass flow multiplied by the speed difference between flight and nozzle speed of the engine. To increase thrust, subsonic engines try to maximize mass flow (by increasing the bypass ratio) while supersonic engines rely more on increasing the nozzle speed (by using afterburners).
The core engines do not differ much - after all, the intake will make sure that air reaches the engine at a speed of Mach 0.4 to 0.5, regardless of flight speed. The main difference is in their bypass ratio. The slower the design speed, the bigger the bypass ratio may become. At very low speed, the ungeared, shrouded fan is exchanged for a geared, free spinning propeller, in other words, the jet changes to a turboprop. The intake and nozzle, however, are very different indeed.
The optimum bypass ratio changes continuously, but since the drag coefficient drops after crossing Mach 1, airplanes are either designed for a maximum Mach number of 0.9 or less, or 1.6 and above. The corresponding bypass ratios today are up to 12 for subsonic engines, and less than 1 for supersonic engines. This produces a sharp boundary at the speed of sound, and many military engines designed for supersonic flight lost their afterburners and were fitted with a big fan to become the engines for subsonic transport aircraft. Examples are:

The Pratt & Whitney J57 turbojet (installed in the F-100 and F-102 fighters, among others) became the JT-3D turbofan which powered the Boeing 707 or the DC-8.
The core of the General Electric F110 (installed in the F-15 and F-16 fighters, among others) became the CFM-56 turbofan which is used in the Boeing 737 or the Airbus A320.

The differences between sub- and supersonic engines grow bigger the more you move away from their core. High-pressure compressor, combustion chamber and high pressure turbine look and work the same, but the low pressure compressor of subsonic engines swallows a lot more air and has a much bigger diameter. Supersonic engines in turn mostly have an afterburner. The biggest difference, however, are the intakes (large pitot intake with blunt lips for subsonic aircraft versus adjustable spike or ramp intakes for supersonic flight) and the nozzle (fixed for subsonic flight versus a complex, adjustable convergent-divergent nozzle for supersonic flight). This is due to the very different air speeds and the much higher exit velocities required for supersonic flight.

Another significant difference is the exhaust nozzle. Subsonic engines use fixed area convergent nozzles, because these are light weight and provide sufficient performance over the generally small nozzle pressure ratios that occur in subsonic flight. Supersonic aircraft need a variable geometry convergent-divergent nozzle, that is heavy, but provides a higher nozzle thrust coefficient over a wider range of nozzle pressure ratios.

@Richard: Didn't the sentence "The biggest difference, however, are the intakes […] and the nozzle (fixed for subsonic flight versus a complex, adjustable convergent-divergent nozzle for supersonic flight)" suffice? I even added a link so people can look up what is special about a con-di nozzle.

Ah...my mistake Peter. Yes, that does suffice, very well. Please accept my (humble) apology.


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Did Boeing build airliners other than the 7x7 series?
What I mean by the 7x7 series is the 707, 717, 727, etc. I took this picture at the Pima Air and Space Museum in Tucson, Arizona.

What type of aircraft is this? Was it an airliner? I know that no 7x7 series aircraft was that small, so were there other airliners made by Boeing besides the 7x7 series?

The aircraft looks a lot like a miniature Boeing 787. Has similar aerodynamic design.

I had to take pictures of this aircraft at the museum, I found it extremely interesting, so I am wondering what it is called, so I could do some research about it.

So, it looks like we already had plans to design the 787 back in the 1950s :)

@RaajTram The shape of it really looks like a mini 787 and the funny thing is that the museum has a 787 to view also. So 2 787's.

@Ethan: I know that no 7x7 series aircraft was that small... the Caravelle (the aircraft pictured) is in fact larger (31m long, 34m span) than the original Boeing 737 (29m long, 28m span). Be careful with absolute statements like that! :)

See Also: this Boeing Frontiers article on Boeing's model numbering scheme

Yes, Boeing built other airliners, with different numbering, although none of them were jet-powered. Three iconic designs come to my mind right away, although there were others:
The Boeing 247 predated the DC-3 into service, and was a highly advanced twin-engine, monoplane cabin design. It was a pioneering aircraft, introducing all-metal construction, a cantilever wing, and retractable landing gear. It was later surpassed by the DC-3, as the 247's cabin was too small for serious airline service.

The Boeing 314 "Clipper" was a large four-engine flying-boat transport from just before World War II. Not quite as pioneering as the 247D, it nonetheless was massive for its time and heavily influenced later large Boeing designs.

The Boeing 377 "Stratocruiser" was a post-World War II design based on the B-29 bomber, with a double-deck (double-bubble) fuselage and four piston engines. It was large, pressurized, and technologically advanced, but relatively unsuccessful in sales compared to contemporary designs like the Lockheed Constellation.

As others have mentioned, the aircraft in your question is a Sud Caravelle, which was designed to solve the same problem as the later Boeing 737: it was a shorter-range airliner meant for smaller routes. You point out how much the Caravelle's nose looks like the Boeing 787, and you're right - but it is even more similar to the De Havilland Comet; Sud licensed the design from De Havilland.
All photos from Wikipedia.

To start with that picture from Pima Air and Space Museum:
It is a Sud SE-210 Caravelle VI-R - the world's first medium-range jet airliner (source: Wikipedia). This particular airplane has nothing to do with Boeing. I believe it's in the Museum due to the fact that it was the world's first medium-range airliner.

You can read all about it at https://en.wikipedia.org/wiki/Sud_Aviation_Caravelle.
To answer your question about whether or not Boeing had anything apart from the 700-series: it does. Not to forget the purchase of MD.
A list of ALL airplanes under Boeing can be found at: https://en.wikipedia.org/wiki/Category:Boeing_aircraft.

I have to ask.. what is that thing hanging down from the underside, just behind the front wheel? spare gas tank? a bomb?

@user2813274 I'm wondering the same. First thought was that it is some sort of a marker beacon antenna, or maybe a glideslope or localizer receiver O.o

This a/c was first flown by United Airlines, and was purchased by Aero Service (as N1001U) which used it as a radar testbed. Hence the pod. More at Airliners.net.

When Boeing purchased MacDonell-Douglas, the only civilian type still in production, MD-90, was renamed to 717, so in that event Boeing was not making any airliners not in the 7x7 series. Boeing did produce many airliner with other numbers before the 7x7 series though.

@JanHudec: Actually, McDonnell Douglas had two civilian aircraft in production when they merged with Boeing in 1997: the DC-9 (in its DC-9-80 [MD-80] and DC-9-93 [MD-90] variants) and the MD-11.  The DC-9-80 and DC-9-93 remained in production until 1999 and 2000, respectively, and a third DC-9 variant, the DC-9-95 (MD-95; Boeing 717) entered production in 1999, where it remained until 2006; the passenger version of the MD-11 was discontinued as soon as the airframes under construction at the time of the merger were completed, but Boeing continued to produce cargo MD-11s until 2000.

There was also Boeing 720, a shorter, smaller version of the Boeing 707. Wikipedia article here: https://en.wikipedia.org/wiki/Boeing_720

All the Boeing civil jet aircraft (which are the only ones under production/operation) are named in the 7x7 series. The military aircraft follow the US DoD designations, like C-17 Globemaster III, K-46 Pegasus (which is a modified 767) etc.
The aircraft is a Sud Aviation SE 210 Caravelle, produced by the French firm Sud Aviation.

"Finnair Caravelle Basle Airport - April 1976" by Eduard Marmet - http://www.airliners.net/open.file/0101623/L/Transferred from en.wikipedia. Licensed under CC BY-SA 3.0 via Commons.
It was the first short/medium-range jet airliner. It first flew in 1955 and was operated till 2004, with around 282 numbers built.

The cockpit windows are different comparing my picture to your is this a different variant.

@Ethan Appears that the cockpit windows have been painted over.


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What is the thing in the middle of the turbofan?
I have been understanding more about engines lately, but I can't figure out what the section in the middle of a turbofan is for:

What benefit does this part provide to rotating the air? Is it some kind of separate fan or something?

By "the weird silver thing", do you mean the hub that all the blades are connected to?

@egid yes I do mean that

@Ethan. Before you edited the question, I thought you were referring to the spirals.

And, before anyone asks why there's a spiral painted on it.

egid is right. It is called spinner.
The spinner basically covers the part of the hub where the compressor blades are attached and reduces drag.

Source: b737.org.uk
It also serves other purposes:

Diverting the airflow efficiently in the center of the engine so that the airflow is smooth over the compressor blade hub

More importantly, the spinner serves to divert the rain and (more importantly) hail impinging on it towards the bypass fan and away from the 'core' of the engine, reducing chances of engine flameout.

The second point an important design parameter for the spinner. According to AGARD-AR-332, Recommended Practices for the Assessment of the Effects of
Atmospheric Water Ingestion on the Performance and Operability of Gas Turbine Engines,

An effective way to remove a sizable amount
  of hail from the core is to optimize the spinner shape
  to deflect hail upon impact directly towards the fan
  duct.

The spinner of the CFM56 engine was modified in response to the engine flameout experienced in Garuda Indonesia Flight 421. The shape was changed into a conelliptical profile.

You can also add that the spiral mark on the spinner is there to produce a spinning white visible circle that scares birds away.

@ Alexus See http://aviation.stackexchange.com/questions/12806/what-are-the-spiral-marks-in-the-center-of-the-engines/12808#12808

That's called a spinner, and it's not that weird! You see them on propellers and turbofans alike.
Spinners are a streamlined shape, and provide aerodynamic drag reduction to what would otherwise be a very draggy, complex part of the powerplant. Here's how a propeller looks underneath:

Perhaps that can explain why streamlining the middle of a propeller (whether a turbofan disc or not) might be beneficial to performance. PeterKämpf would better be able to address the aerodynamics, but I am under the impression that cleaning up the airflow at the center of the propeller or fan improves efficiency of the blades as they are encountering less turbulent airflow.


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Do fly-by-wire flight controls in airliners provide artificial feel?
Mechanical and hydro-mechanical flight control systems (cables, pushrods, hydraulics) all offer pilots some form of feedback – direct or, in the case of hydraulics, artificial – as the aircraft maneuvers and forces are applied to the controls.  I know that early FBW, like that in the F-16 prototypes, used a rigid system that often allowed pilots to over-control the aircraft.
Do fly-by-wire-equipped airliners, like their non-FBW siblings, use an artificial feel system to provide control feedback?

After some digging, I found an answer for myself: yes, some FBW airliners use artificial feel systems, but not all of them. According to Electronics in the Evolution of Flight (Google Books),

The [Boeing] 777 fly-by-wire system employs envelope protection. This feature of the artificial-feel system provides increasingly greater force when the aircraft is pushed to its limits.

Conversely, Airbus does not provide any feedback to the pilots as their flight control system enforces control laws that cannot be overridden by inputs (source: Aerospatiale engineering document):

The positioning of the control surfaces is no longer a simple reflection of the pilot’s control inputs and conversely, the natural aerodynamic characteristics of the aircraft are not fed back directly to the pilot.

Getting regulatory, since I did some further research: 14 CFR 29.395, certification of Transport Category aircraft, only states that the controls are required to resist a certain amount of force (i.e., not break). This differs from how much force is required to "achieve the positive limit maneuvering load factor" or otherwise impart changes to the aircraft's orientation (14 CFR 23.155 / 23.157).

This should be:  "Conversely, Airbus does not provide any feedback to the pilots as their flight control system normally enforces control laws that cannot be overridden by inputs..."

I would actually argue that the simple spring loading of Airbus side-stick provides exactly the feel that it should except when you hit the flight envelope limit.

Envelope protection is different than an artificial feel to the controls.

14 CFR Part 29 covers Transport Category Rotorcraft.  Part 25 covers Transport Category Airplanes. But even Part 25 does not specify forces other than an allowable maximum force for the different controls. §25.397   Control system loads.

Sorry but the quoted source does not say that Airbus does not provide any feedback. The quote is in the context that by removing the mechanical connection there is no direct feedback from the surfaces to the controls. As Jan mentions the spring loading and profile of the stick will provide artificial feedback to the pilot.

Yes FBW aircraft use an artificial feel system to provide control feedback. Artificial, as in: there is no feedback to the stick from the airflow forces on the control surfaces, and the stick would feel very light if not loaded by some sort of mechanical feel spring. The mechanical feel spring makes the feel artificial; the spring can have a constant stiffness, or a varying stiffness as function of airspeed (q-feel).
All FBW must have artificial force feel because there is no direct mechanical connection between the control surface and the stick. Also all irreversible flight controls must have artificial force feel: although there is a mechanical link between stick and surface, aerodynamic moments on the control surfaces are not fed back through the hydraulic actuators that move the surface. The extent of the feedback feel depends on the manufacturer:

Airbus was the first airliner manufacturer to introduce fly-by-wire, in the A320. They chose for uncoupled sidesticks for pitch & roll inputs, with passive mechanical spring/dampers providing a force proportional to stick deflection and velocity.
Boeing first implemented FBW in the B777. This aircraft has conventional wheel/columns for pitch and roll inputs, not connected by cables to the elevators/ailerons, but loaded by an active artificial feel system to provide feedback forces on the stick. The feedback forces are provided by a hydraulic actuator and varied according to aircraft state, resulting in similar force characteristics to earlier Boeing airliners. The B737 also has artificial q-feel in the elevator, through a system that changes the mechanical advantage of the mechanical feel spring

Note that both aircraft function the same in that they both use control deflections for inputs for the flight computers, which compute the desired surface deflection. Only the stick coupling and the feedback forces differ. From the B777 FCOM:

Elevator Variable Feel
The PFCs calculate feel commands based on airspeed. In general, control column forces increase:

as airspeed increases for a given column displacement, or
as column displacement increases.

The second bullet point describes a simple spring characteristic. The first bullet point means that the spring stiffness varies with airspeed, like when the elevator would be reversible and the aeroforces could be felt directly at the stick. The A320 does not have a variable spring gradient and spring forces are always the same, regardless of flight state. It is still an artificial feel though: all irreversible flight controls have artificial feel, not all of them have artificial q-feel.

To answers this question I need to remember my favourite subject, Principles of Flight - just kidding. The key to the aswer is the requirement that civil (sorry, I don't know about military, but you're also not asking) aircraft are required to have a certain amount of stick force per G. What does this mean? In simple words:the aircraft is required to require the pilot to put more force on the controls if he wants to achive a higher load factor. Even simpler: we are supposed to work hard to have fun ;)
Now we need to know what fly by wire is. FBW means that the imputs from the pilots are brought to the control surfaces, or better the actuators digital. So there is no mechanical connection between the surfaces, where the aerodynamic forces act and the controls the pilot feels. That means that there are also no stick forces per G. That would also make a pilots life very hard. I can not imagine to fly a plane without feeling it / getting a feedback on the controls.
Taking into account both facts I've listed above my answer is yes, an airliner which is fly by wire equipped needs also to be equipped with an artificial feel system.

Do you have any sources for the civil aircraft requirement? Is it in the U.S. FARs? Why would Airbus be exempt from this? I agree that it seems like no feedback would cause problems, but Airbus pilots seem to love how their aircraft fly.

I honestly have no idea of flying an airbus, but as far as I know these aircraft are equipped with some kind of artificial feel systems. Sorry, I can't provide you with any reference to the FARs, also not to EASA, but I'm sure about the EASA rules.

Airbus controls have some give to them (the stick moves, with some resistance) but the system does not have a feedback loop - there is no artificial feel system, at least according to an article I found by some Airbus/Aerospatiale engineers... :)

This should be a trustworthy source. So airbus maybe fulfilled the need for certain stick force per G by these resistance in combination with the logic of their controls - all in all the result would be quiet the same. I need to try these planes :D

Yes, what @egid said is true. Airbus fly-by-wire aircraft only have a sort of spring load system which increases the force required based on how far away the sidestick is from the centre position, so no force-feedback there.

Spring loading is force feedback: our hand feels the spring force. There is no aircraft load factor feedback into the flying control force.


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Why does this B-52 carry a payload only on its right wing?
Here is a picture of a B-52 located at the Pima air and space museum.

Source:(www.airplaneexhibits.com)
As you can see in the picture on the right wing next to one of the engines there is some kind of missile carrier, but the left wing has no missile carrier. Wouldn't the weight of the aircraft be very unbalanced with a missile on the right side, but no missile on the left side. I have noticed this in a lot of pictures of B-52's where they are carrying a missile on the right wing, but no missile on the left one.

This particular missile was called North American X-15. It was indeed rocket-powered, but also hat a pilot onboard. See here for more.

Is that photograph's left or plane's left?

@DJohnM From looking at the picture it on the left side, but its really on the right side.

@Peter is right.
The aircraft in question is the "The High and Mighty One" a NB-52A serial number 52-0003, which was used as a launch aircraft for the North American X-15, a hypersonic rocket powered aircraft. The 'missile carrier' (pylon) is used for carrying the X-15.
Here is a view of the carrier itself, just after launching.

"X-15 launched bw". Licensed under Public Domain via Commons.
This was used as the X-15 was not designed to takeoff under its own power; it was rather designed to be carried aloft and drop launched from under the wing of a NASA B-52 'mother ship'. As of March 2016, the X-15 holds the official world record for the highest speed ever reached by a manned, powered aircraft.

Image from air-and-space.com; Photo courtesy North American Rockwell.
The aircraft underwent significant modifications for installation of the pylon, including removal of the wing (no. 3) fuel tank, disabling the inboard flaps in the starboard wing and cutting a notch to accommodate the X-15's vertical stabilizer.
The heaviest weight the aircraft carried on the pylon was ~53,100 lb, for X-15 with external fuel tanks. A number of tests were carried out and it was found that the carrier aircraft were capable of trimming out the unbalance of the B-52/X-15 combination. So this caused few problems in flight in the operational range.

I would imagine that the weight of the X-15 was nearly offset by the missing #3 fuel tank.

@FreeMan But then, on the launch, you get some significant side-kick :)

@yo' that's true, then when it's gone, you've got all that weight missing and no wing tank to pump fuel into to balance things back out. But, as the answer said, they were able to trim it back into balance, so it was all good.